Two-wire constant current powered transducers are presently using individual Field-Effect transistors (FETs) and Bipolar Junction transistors (BJTs) to process data. These circuits are heavily dependent on the characteristics of the active components, which usually have a wide range of component tolerance, such as gain, bias voltage, bias current and temperature coefficient.
Accordingly, each production unit has to be initially calibrated to incorporate those tolerances. Moreover, the optimal operating point drifts widely as a function of both temperature and age. The bandwidth is narrow (because of not having enough gain). The design is thus difficult and the labor and parts cost are high.
The following prior art reflects the state of the art of which applicant is aware and is included herewith to discharge applicant's acknowledged duty to disclose relevant prior art. It is stipulated, however, that none of these references teach singly nor render obvious when considered in any conceivable combination the nexus of the instant invention as disclosed in greater detail hereinafter and as particularly claimed.
______________________________________ INVENTOR ISSUE DATE U.S. PAT. NO. ______________________________________ Bruckenstein et al. November 4, 1975 3,918,005 Barr December 11, 1979 4,178,525 Peters March 1, 1988 4,727,752 Park et al. February 28, 1989 4,807,482 Alexander March 17, 1992 5,097,223 Verheyen et al. July 14, 1992 5,130,598 ______________________________________
U.S. Pat. No. 3,918,005 to Bruckenstein, et al. teaches the use of op-amp circuitry with automatic self-biasing wherein either a controlled current or voltage is provided at the load impedance Z connected across the inverting input and the second op-amp A2.
U.S. Pat. No. 4,178,525 to Barr teaches the use of a two-wire piezoelectric acceleration transmitter for detecting and transmitting peak amplitudes of cyclic voltage developed by transducer 32 responsive to vibration.
U.S. Pat. No. 4,727,752 to Peters discloses an oscillator drive system for a piezoelectric crystal 20 whose output corresponds to a nearly sinusoidal resonator 12 signal and is illustrated as being used in an accelerometer. Peters discloses the application of driving the crystal with a digital signal where the oscillator is shown FIG. 4. Alternative embodiments are disclosed.
U.S. Pat. No. 4,807,482 to Park et al. discloses a method for measuring stimuli applied to a piezoelectric transmitter. Two embodiments are disclosed.
U.S. Pat. No. 5,097,223 to Alexander teaches the use of a current feedback audio power amplifier utilizing a pair of op-amps A1 and A2 wherein A1 is a buffer amplifier used for audio gain and A2 is used for sub-audio gain. The output of A1 is connected in a voltage gain feedback circuit back to its inverting input and a voltage gain feedback loop is connected across A2. A current limiting circuit can be provided if desired to limit the current in the output stage.
U.S. Pat. No. 5,130,598 to Verheyen et al. discloses apparatus for driving a piezoelectric actuator for a fuel injector.